Air Safety Link—EMB-135/145 Pilots Warned on Pitch Trim

Air Line Pilot, May 2001, p.6
By Jan W. Steenblik, Technical Editor

ALPA has strongly urged pilots of the Embraer EMB-135 and –145 to obtain a copy of the latest operational guidance regarding Embraer EMB-135/145 pitch-trim anomalies from their airline and to comply with the guidance until further notice. The Association has also put out the call for pilots to report any pitch-trim anomalies in these airplanes to ALPA.

In ALPA Operations Bulletin 2001-02, issued March 26, Capt. John Cox (US Airways), ALPA’s Executive Air Safety Chairman, warned that the FAA’s Aircraft Certification Office (ACO) has learned of at least 10 incidents of EMB-135/145 pilots having limited elevator authority to overcome pitch-trim input. These incidents have occurred since Embraer modified the bellcrank in the EMB-135/145 elevator-control system approximately 2 years ago to reduce control-column forces during a Flaps 18 takeoff.

"If the pilot fails to trim before reaching a certain speed, air loads may overpower the horizontal stabilizer control actuator, and the horizontal stabilizer may slip and subsequently stall," the ALPA bulletin warned. "The flight crew may not receive any immediate visual indications of this type of pitch-trim failure, except for extremely high pitch-control forces or limited elevator authority."

On Dec. 27, 2000, the pilots of an EMB-135 experienced a serious trim-system anomaly that resulted in their having limited control of the pitch of the airplane. During climb, the airplane’s pitch-trim system failed to operate, remaining at the takeoff trim position.

The flight crew was unable to overcome the nose-up tendency caused by the trim system, in spite of keeping the control columns in the full forward position. Only after reducing airspeed and lowering the landing gear were the pilots able to regain some pitch control and subsequently land the airplane. The NTSB is investigating the incident, and ALPA is a party to that investigation.

On Jan. 19, 2001, the FAA issued Emergency Airworthiness Directive (AD) 2001-02-51 to all U.S. operators of EMB-135s and –145s. This AD requires revising the FAA-approved airplane flight manual and installing placards to alert the flight crew to the maximum airspeed for retrimming after takeoff and during climb. The actions specified in this AD are intended to prevent high pitch-control forces, which could result in loss of control of the airplane.

Embraer has flight-tested this scenario and conducted some analysis, and has determined that this phenomenon occurs at approximately 180 to 190 KIAS, about the time the pilot begins the initial climb schedule. The manufacturer believes that if the takeoff trim setting (approximately 7–8 units nose up) is maintained until reaching 200 knots, the loads on the tailplane may be higher than the horizontal stabilizer can handle and the actuator would slip and stall with little, if any, early indication to the flight crew.

Embraer and the FAA are recommending retrimming the EMB-135/145 after takeoff and before reaching 160 KIAS. In addition, Embraer has produced, for installation at both flight crew stations, a placard that states this restriction. The FAA believes that this temporary fix should keep the airplane out of the area of the flight envelope where a pitch-trim system anomaly has been identified and a limited elevator-authority condition can—and has—occurred.

Embraer has planned several upgrades to trim switches, aural warnings, and the EICAS that were to be installed on these airplanes beginning in April. Embraer also is about to begin a fleetwide inspection of all horizontal-stabilizer actuators.

The manufacturer is continuing to explore the possibility of making a permanent design change to the pitch-trim system to preclude this type of event happening again. To date, however, Embraer has not succeeded. The FAA ACO is working with Embraer to try to fix the problem.